Bluebook: Is the Period in Id. Italicized?

When you cite with Id., italicize the period as well as the two letters. Bluebook Rule 4.1 states, "Note that the period at the end of 'id.' is always italicized."^[1]

Write Id. at 563, not Id. at 563.

Why you can't see it

In Times New Roman, the italic period looks the same as the roman one. On October 17, 2026, Claude, an AI model made by Anthropic, measured the two periods in Liberation Serif, a free font made to Times New Roman's character widths. Both are the same round dot, the same size and the same width. The italic dot sits about 1/40 of an em further left, which in 12-point type is about a third of a point, or a tenth of a millimeter. On the page, the difference is invisible.

So the only way to see it is to look at the codes behind the page, or to show italics with something other than slant, such as a color or a highlight.

Where you can see it

Markdown

Right: *Id.* at 563.
Wrong: *Id*. at 563.

HTML

<i> works the same as <em>:

Right: <em>Id.</em> at 563.
Wrong: <em>Id</em>. at 563.

To see it in a browser, give italics a background color:

em { background: #fff3a0; }

The highlight covers the period only when the period is inside the <em>.

LaTeX

Right: \textit{Id.} at 563.
Wrong: \textit{Id}. at 563.

Claude also compiled both LaTeX lines in Times and read the fonts in the PDF. In the first line, the period comes from the italic font. In the second, it comes from the roman font. Both periods are the same width, so nothing on the page moves.

WordPerfect

In Reveal Codes: italics is a paired code, named Italc. "Paired codes have an On code and an Off code," and "[t]hese codes surround the text that is affected by them."^[2] Reveal Codes draws the On code as a tag with its point on the right, toward the italic text, and the Off code as a tag with its point on the left, back toward it. Spaces show as small diamonds. Corel's own tutorial shows a pair of Italc codes around "Alt+F3," with the comma after the Off code, so the comma is not italic.^[3]

The period of Id. is italic only if it sits between the two codes. In plain text, with > and < standing for the points of the tags:

Right: [Italc>Id.<Italc]◇at◇563.
Wrong: [Italc>Id<Italc].◇at◇563.

Checking Markdown with Python

A regular expression finds each Id. For each one, the script counts the asterisks before it on the same line. An odd number means italics are open, so the Id. is italic.

import re
import sys

OUTSIDE = re.compile(r"\*([Ii]d)\*\.")   # *Id*. -- the period sits outside the italics
BARE = re.compile(r"\b[Ii]d\.")          # Id. or id.

def check(line):
    problems = []
    for m in OUTSIDE.finditer(line):
        problems.append("period outside the italics: " + m.group(0))
    for m in BARE.finditer(line):
        # Italics are open when an odd number of * comes before it on the line.
        if line[:m.start()].count("*") % 2 == 0:
            problems.append("not italicized: " + m.group(0))
    return problems

for number, line in enumerate(open(sys.argv[1], encoding="utf-8"), 1):
    for problem in check(line):
        print(f"line {number}: {problem}")

Save it as check_id.py and run python3 check_id.py post.md. On a file with these lines:

[^n1]: *Id.* at 563.
[^n2]: *Id*. at 563.
[^n3]: Id. at 563.
[^n4]: *See id.* at 563.
[^n5]: *See* id. at 563.

It prints:

line 2: period outside the italics: *Id*.
line 3: not italicized: Id.
line 5: not italicized: id.

The script has three limits. It reads * as the only italic marker, not _. It does not skip code blocks, so it flags the wrong examples in this post. And it flags a lowercase "id." anywhere, including ordinary prose such as "each file has an id."